Sodium ions are essential for the
functions of biological
cells,
and they are maintained at the balance between intra- and extracellular
environments. The quantitative assessment of intra- and extracellular
sodium as well as its dynamics can provide crucial physiological information
on a living system. 23Na nuclear magnetic resonance (NMR)
is a powerful and noninvasive technique to probe the local environment
and dynamics of sodium ions. However, due to the complex relaxation
behavior of the quadrupolar nucleus in the intermediate-motion regime
and because of the heterogeneous compartments and diverse molecular
interactions in the cellular environment, the understanding of the 23Na NMR signal in biological systems is still at the early
stage. In this work, we characterize the relaxation and diffusion
of sodium ions in the solutions of proteins and polysaccharides, as
well as in the in vitro samples of living cells. The multi-exponential
behavior of 23Na transverse relaxation has been analyzed
according to the relaxation theory to derive the crucial information
related to the ionic dynamics and molecular binding in the solutions.
The bi-compartment model of transverse relaxation and diffusion measurements
can corroborate each other to quantify the fractions of intra- and
extracellular sodium. We show that 23Na relaxation and
diffusion can be used to monitor the viability of human cells, which
offers versatile NMR metrics for in vivo studies.